Measurement technology is constantly faced with the problem that measurements cause feedback to the device under test. As a result, measuring instruments are designed to keep this feedback as small as possible: voltage meters have a high input impedance; power meters have an impedance value which is, if possible, as large as the rated impedance of the reference system (for example 50Ω). While it has been possible for a long time to come very close to the ideally-required impedance value in the low-frequency range, that is to say, to operate almost without feedback, this is possible only to a limited extent in the high-frequency and microwave range. For example, the design-determined parasitic input capacities of oscilloscope probe heads have the effect that the impedance value in the GHz range falls below the 1 kΩ-level. For example, in the case of microwave power sensors, it is the mechanical tolerance of the connecting jacks, which set a lower limit for the attainable matching.
Power sensors (or more accurately, terminal power sensors) are used to measure the rated power of a source, that is to say, the power, which the source can emit with surge-impedance-corrected matching.
Accordingly, power sensors are designed in such a manner that their input impedance deviates as little as possible from the reference surge impedance (generally 50Ω), and that they display the power supplied. However, since a certain mismatching of the power sensor cannot be avoided, this can impair the accuracy of power measurements.
On the one hand, mismatching on the part of the sensor means that some of the power supplied to it is emitted again in the form of a reflected wave. In the case of a thermal sensor, for example, this would have the effect that the net heat convertible into power is less than the power supplied to the sensor. In order to correct this effect, power sensors are already adjusted (calibrated) in such a manner that they display the power of the incident wave independently of the level of power actually supplied to the measurement converter. Expressed in simpler terms, a frequency-dependent sensitivity S is determined, which links the raw measured value (e.g. the output value X of a thermoelectric measuring cell) and the power of the incident wave:
                    S        =                  X                                                                  a                i                                                    2                                              (        1        )            ai incident wave|ai|2 power of the incident waveS sensitivityX raw measured value
Given an accurate calibration process, it can therefore be assumed that the effect of the mismatching on the sensor itself can be corrected, that is to say, the power of the incident wave can be measured with a low uncertainty.
However, a second effect linked to the mismatching of the power sensor cannot be so easily reversed. The wave reflected back to the source can be reflected there again and can be superimposed on the wave incident on the power sensor, so that the power of the source changes, and the source no longer emits its rated power (feedback of the sensor to the device under test). The level of the deviation is determined by the value and phase of the reflection coefficient of the sensor and the source. The context is described by the following mathematical relationships:
                                                                    a              i                                            2                =                              P                          GZ              ⁢                                                          ⁢              0                                                                                        1                -                                                      Γ                    G                                    ⁢                                      Γ                    S                                                                                      2                                              (        2        )                                          P                      GZ            ⁢                                                  ⁢            0                          =                              X            S                    ·                                                                  1                -                                                      Γ                    G                                    ⁢                                      Γ                    S                                                                                      2                                              (        3        )            PGZ0 rated power of the source (with matching)ΓG complex reflection coefficient of the sourceΓS complex reflection coefficient of the power sensor
The measurement deviations caused by the feedback are generally negligible at low frequencies, where small reflection coefficients can easily be realised. However, in the microwave range, measurement deviations can occur, which exceed the specified uncertainty of the sensor for the incident wave several times over. For example, with a sensor with a reflection coefficient of 0.13 (specification for R&S NRP-Z55 at 40 GHz) and a source with a reflection coefficient of 0.33 (SWR 2.0), a measurement deviation within the range from −8.9% . . . +9.3% can occur. This is significantly more than the measurement uncertainty specified for this sensor (for the incident wave) at the level of approximately 2.5%.
Two methods are known for reducing the influence of feedback.
1. The resulting measurement deviation is reversed retrospectively using equation (3). This presupposes that the complex reflection coefficients of the source and the power sensor are known with reference to value and phase (real and imaginary component). For the power sensor, this requirement is easy to fulfil, because the reflection coefficient must be measured anyway within the framework of the production process and must also often be documented. For example, hitherto, the applicant has stored the complex reflection coefficients for the sensors in the NRP range of power meters in the data memory of the sensor and can therefore offer the correction option explained above. For this purpose, however, the reflection coefficient of the source must be entered, that is to say, it must have been measured previously. This is the actual problem of this method. Apart from the fact that it may be troublesome to measure this parameter, if no network analyser is available or if no time is available for a measurement of this kind, this measurement cannot readily be implemented at all using standard testing methods, because the reflection coefficient of a source is involved. There are, in fact, also methods available for this purpose, but it cannot generally be assumed that these methods are known and/or that the necessary equipment is available. In summary, this correction method is only used in cases, in which the increased expenditure is really worthwhile.2. Using a tuner connected upstream of the sensor, the reflection coefficient of the sensor is adjusted at the measurement frequency to zero, so that no measurement error can occur as a result of mismatching. In this case, a network analyser is also required, in fact, for the adjustment of the tuner. An additional difficulty with this method is that the tuner adjustment is frequency dependent, that is to say, the tuner must be readjusted each time for power measurements at different frequency points. Accordingly, a remote-controllable tuner is almost indispensable for practical implementation. This method is used in isolated cases in calibration laboratories, where the measurement accuracy attainable is comparable with that described above under 1.